Electroacoustic transducer



Jun 22, 1943. l F, R, FARROW, JR 2,322,621

ELECTRO-ACOUSTIC TRANSDUCER Filed Oct. 2, 1941 l messed June 22,1943

r UNITED STATES- PATENT oFFlcs-f l "azzzsz'i," y l ELECTBOACQUSTIC TBANSDUCER Frederick a.- Fmow, Jr.. Audubon, N. J.. minor to Philco Radio and 4Television Corporation, Phiiadelphixala., a corporation of Delaware Application october z, im, serial No. 413,363

(ci. 17a-local) 11 Claims.

This invention relates t'o electro-acoustic transducers of the ty'pe adapted when actuated by an acoustic system, such as a phOnOslaPh recording, microphone, or like device, to control anl electrical system such as an electric wave signal generator or amplifier. 'I'he invention is particularly adapted for use in phonograph pickl ups or microphones in which power supplied from an acoustic system to a -transducer operates the frame are shown apart so as to reveal the operative members. Each part 6 and 1 has a rectangular central opening 3. a at sided front bearing yoke part 3 and a ilat rear yoke part I0. The

yoke parts 3 form a rectangular or square bearing. The operative members of the reproducer comprise two metallic strips or ribbons II and I2 separated throughout their length by a strip of insulating material I2a. vWhen the parts are assembled, the rear ends of ribbons II and I2 are clamped in position by the yoke parts I0, while a novel variable condenser directly controllable" by a suitable acoustic system and adapted to serve as a variable tuning element in a wave signal generator. L Y

Another object of the invention is to provide a novel variable condenser directly operable by the stylus of a phonograph reproducer.

Still another object of the invention is to provide a novel phonograph pickup particularly A adapted for association with a wave signal generator capable of generating a frequency-modulated carrier wave.

The invention itself, as well as other objects thereof, may be clearly understood by reference to the accompanying` drawing, wherein Fig.` 1 is a sideelevation of a tone arm adapted for use in th'e reproduction of disc recordings, with a portion broken away to show the reproducing element; 4 Fig. .2 is an exploded perspective view ofthe reproducing element;

the front ends of ribbons I I and I2 are held by a U-shaped clamp I3, there beingsuitable insulating material I4 between the ribbons I I and I2 and the frame parts 6 and 1 and clamp I3. As shown, y

clamp I3 is mounted on one end of a rectangular or square shaft I5. An arm I6 is secured to the other end of shaft I5 and has three'angularly oil'- set portions suchthat the free end may carry a stylus I1 adapted 4to ride in the groove of a record,- ing. The arm I6 may be constructed in accordance with the disclosure in my copending application Serial No. 390,822, filed April 28, 1941. resilient bushing I8'of rubber or the like surrounds shaft IS and is clamped in the yoke parts 9 to substantially secure shaft I5 against longitudinal movement, while permitting slight rotational movement. The bushing I8,l with yoke'.

parts 9, serves to restore the shaft I5 to its normal angular" position whenever it is displaced therefrom. together by screws or the like, and the assembled reproducer is mounted' on the tone arm by bolts 5.

When the parts are assembled, asabove described, the arm' I6 normally'extends substanl tially downward and clamp I3 is positioned at an acute .angle with respect to the rear yoke parts I 0 of frames 6 and 1, so that the ribbons II and I2 Fig. 3 shows one suitable electrical circuit which may be used with the reproducing element; and

Fig. 4 shows a preferred form of the reproducing element.

Referring to Fig.1, there is shown a tone arm I adapted to be pivotally mounted adjacent a turn-l table (not shown) for movement thereover during the reproduction of a recording. s A vertical post 2 may serve as the tone arm support. At its free end tone arm I has an enlarged head portion 3 within which a reproducing element 4 may be secured as by fastening means 5.

'I'he reproducing element is best shown4 in Fig. 2 wherein the upper and lower halves I and 1 of a `are normally twisted about their longitudinal i ax'es as illustrated. These ribbons may be permanently deformed or they may be retained in their twisted position under initial stress. Electrical contacts I9 and 20 may be secured to ribbons II and I2 and wires 2| may extend from these contacts' through the tone arm to suitable circuits.

'I'he above described reproducing element constitutes an electrical condenser composed of the ltwo ribbons II and I2 spaced by the insulator orv dielectric I2a, and when the plates are in their normal position, as shown, they will constitute a condenser oi' predetermined capacity.

As the stylus I1 traverses the sound groove of a recording, the crank arm Il will translate the lateral displacement ofthe stylus by the record The clamping frames 6 and 1 are secured into an oscillatory motion of shaft I5. By virtule of this, the ribbons I I and I2' will be twisted dr torsionally flexed about their longitudinal axes, the oscillations -of arm Il tending when the arm is moving in one direction, to increase the twist of the ribbons, and when moving in the other direction, to decrease their twist. This movement of the ribbons will cause them to move toward and away from each other throughout their length by a very small amount, thus changing the spacing of the ribbons, it being understood that they are not clamped together except at their ends. This movement of ribbons I and II relative to each other will, of course, vary the capacity of the condenser invaccordance with the lateral deflection of the stylus by a recording.

In Fig. 3 there is shown by way of example one suitable electrical circuit for use with the reproducing element above described. The circuit comprises a radio receiver of the superheterodyne type, wherein a common oscillator circuit is employed both as the local oscillator of the superheterodyne during radio reception and as the phonograph pickup oscillator during phonograph operation substantially as disclosed in the application of R. B. Albright, Serial No. 397,986, ied

June 13, 1941. The radio receiver may, comprise a radio frequency amplifier 22, a detector-oscillator stage including the tube 23, an intermediate frequency amplifier 2l, a second detector, and A. V. C. stage 25, an audio frequency .amplier 26, and a loudspeaker 21. When the switches 28, 29 are in'their lefthand position, the receiver is adapted for radio reception. When the switches 28, 29 are in the position shown, the

phonograph pickup oscillator tank coil 30 is connected to the oscillator grid and oscillator anode of the tube 23. The coil 30 may be tuned to resonate at a desired frequency by a fixed condenser 3 I aA small adjustable condenser 32', and `the condenser 33, which corresponds to the condenser formedby ribbons II and I2 of the reproducen' I above described. Coil 30 is preferably tuned to a frequency above or below-the intermediate frequency at a point on the I. F. selectivity characteristic where the characteristic is linear over an appreciable range of frequencies. f If now the frequency of the phonograph oscillator is caused to vary about its mean frequency, due to the actuation of condenser 33, the radio frequency output of the I. F. amplifier will vary in amplitude,

and the output voltage of the second detector will vary in accordance with the audio frequency signal of the recording. This mode of operation of the receiver circuits is more fully described in. the said Albright application and forms no part of the present invention.

Preferably a constant capacity cable, such as; the coaxial cable 34, is employedto connect the pickup condenser 33 to coil l0. Since'the capacity variations of condenser 33 are very small, it

` is highly desirable that there -shall be nol extraneous'capacity variations due to the connecting leads. The use of a coaxial 'cable prevents such extraneous variations.

' One embodiment of the present 'Inventionl applied to a phonograph pickup device employed .005 inch brass condenser ribbons separated by .0025 inch mica-insulation, the ribbons being twisted approximately 25 degrees about their longitudinal axes.v`

Other materials or dimensions may be used, such as glasscloth yinsulation or y a resilient insulator, such as latex, and 'many structural 'modifications are possible without departing from the invention.

In l

inner ribbon as illustrated. Dielectric sheetsL '3l serve to insulate the two ribbons. With this. preferred form of the device, the outer ribbon 36 may contact the frame parts 6 and 1 and is preferably connected to an electrical ground, as by the shielded lead 34, whereby pickup of stray fields is eliminated. Further, in this device, the strip 36 may be attached directly to shaft I5.

A transducer of the type herein described is known as lan active transducer and may be de- Iined as one in which the applied power from a recording modulates local power supplied from the oscillator. This type of transducer is, of course, distinct from transducers of the passive type, such as those using piezo-electric crystals -in which the output power is derived solelyfrom the applied power. Active transducers or modulators have particular advantages when employed for phonograph reproduction in conjunction with radio receivers, in that they permit the use of the high frequency amplifying circuits of the receiver during phonograph reproduction, and it will be clear that the present invention provides an essentially ideal transducer permitting. full advantage to be taken of its inherent Vcharacteristics of simplicity, dependability, and quality.

I claim:

1. In an active electro-acoustic transducer, a

variable condenser comprising spaced insulated flexible metallic sheet members, said members having substantially rectilinear 'axes and having va predetermined amount of. pre-twist about said axes, and means adapted to torsionally deflect said members about said axes to vary the capacity of said condenser.

2. In an active electro-acoustic transducer, a variable condenser comprising closely-spaced insulated flexible metallic strips pretwisted about their longitudinal axes, and means adapted to torsionally defiect said strips about said axes to vary the capacity of said condenser.

3. In an active electro-acoustic transducer, a variable condenser comprising spaced insulated flexible flat metallic members, said members having substantially parallel rectilinear axes and. having a predetermined amount of pre-twist 5. In a phonograph pickup device, a tone arm.

a pair of oppositely facingfframe members secured to said tone arm, said frame members each having a central opening bounded at opposite ends by yoke portions, vibratory means extending into said opening comprising a pair of insulated iiexible metallic members each having one end rigidly clamped between one pair of said yoke portions, a `resilient bearing secured'betvgeen the other pair of said yoke portions, a vibratiy shaft extending through said bearing, said shaft; being secured to the other ends of said metalli-'inembers, and` means comprising a stylus arm and stylus for actuating said shaft to torsionally desaid members relative to each other.

6. In a phonograph pickup device, a tone arm, a pair of oppositely facing frame members secured to said tone arm, said frame members each having a central opening bounded at oneend by a ilat yoke portion and at the other end by a recessed bearing-forming yoke portion, vibratory' means extending into said opening comprising a pair of closely-spaced insulated exible metallic members pretwisted about their longitudinal axes, each of saidl members having one end rigidly clamped between the pair of iiat yoke portions, a resilient bearing secured between the other 'of said yoke portions, a vibratory shaft exclamp means secured on said shaft adapted to secure the other ends of said metallic members to said shaft, and a stylus armand stylus for actuating said shaft to torsionally deflect said members and thus vary the spacing of said members relative to each other.

7. In a phonograph pickup device including a tone arm, a reproducer mounted on said tone arm comprising a condenser formed of two relatively movable long narrow metallic strip elements normally twisted about their longitudinal axes, one of said elements enveloping the other of said elements, and means for deecting both of said elements relative to each other about said axes in accordance with a recording to vary the capacity of said condenser.

8. In an electro-acoustic transducer, a vari able condenser comprising a pair of closely spaced insulated long narrow flexible metallic strips normally twisted about their longitudinal axes, one of said strips enveloping the other of said strips, and means comprising a pivotally mounted vibratory stylus arm adapted to torsionally deflect said strips about said axes to vary the capacity of said condenser.

`tending-through said bearing into said opening,

9. In an electro-acoustic transducer, a variable condenser comprising spaced insulated nexible metallic elements, means for rigidly supporting` certain portions of said elements, means for supporting other portions of said elements so as to permit torsional deflection thereof about an axis extending between said portions, said elements having a predetermined amount of pretwist about said axis, and means for torsionally deflecting said elements about said axis to vary the capacity of said condenser.

l0. In a phonograph pickup device, a tone arm, a pair of oppositely facing rigid members secured to said tone arm, a variable condenser comprising spaced insulated ilexible metallic elements each having a portion rigidly clamped between cooperatinsparts of said members, means includ-A ing other cooperating parts of said members for supporting other portions of said elements so as to permit torsional deflection of the latter about an axis extending between the said parts of said members, said elements having a predetermined amount of pre-twist about said axis, and means including astylus for torsionally deecting said elements about said axis.

l1. In aphonograph pickup device, a tone arm, a pair of oppositely facing rigid members secured to said tone arm, a variable condenser comprising spaced insulated flexible metallic elements each having a portion rigidly clamped between cooperating parts of said members, a non-circular shaft secured to otherportions of said eley ments, a non-circular resilient bushing on said shaft, a yoke formed by other parts of said members adapted to clamp said bushing, said elements when thus supported by said members being twisted about an axisextending between said parts of said members, and astylus secured to said shaft for torsionally deilecting said elements about said axis.

FREDERICK R." FARROW, Jn. 

